The skin is a complex metabolic organ with unique structure and function (M. W. Greaves, “Physiology of Skin,” J. Invest. Dermatol. 67(1), 66-69 (1976)). It is composed of three primary layers, namely the epidermis, dermis, and subcutaneous. The epidermis comprises the upper or outer layers of the skin, is nonvascular, and varies in thickness over different parts of the body. The epidermis itself is composed of several different layers, specifically the stratum corneum, stratum lucidum, stratum granulosum stratum spinosum, and stratum basale layers.
The uppermost or outermost layer of the skin is the stratum corneum, also known as the “horny layer” of the skin, is composed mainly of dead cells that lack nuclei, the uppermost of which slough off over time. The cells within the stratum corneum are flat and scale-like in shape. These cells, composed mainly of the protein keratin, are arranged in overlapping layers, imparting a tough and hydrophobic nature to the stratum corneum.
Below the stratum corneum is the stratum lucidum, a homogeneous translucent band, much thinner than the layers above and below it. Below the stratum lucidum layer of the epidermis is the stratum granulosum, composed of two or three rows of flat cells composed mainly of keratohyalin, which is transformed into keratin in more superficial layers. Stratum spinosum lies below the stratum granulosum and is composed of several layers of polygonal cells known as “prickle cells.” The number of layers of cells in the stratum granulosum varies over different regions of the body.
Below the stratum spinosum layer is the stratum basale layer, also known as the stratum germinativum, the deepest layer of the epidermis. The stratum basale is composed of columnar cells which are continually dividing to produce new skin cells. It is the cells in the stratum basale that produce melanin. Over time, the cells produced in the stratum basale move upward and away from the blood supply, and their cell contents and shapes change, forming the different layers of the epidermis. Under normal conditions, the basal layer cells migrate upward over the course of two weeks to create the stratum spinosum and stratum granulosum. An additional two weeks elapses before those cells are exfoliated from the stratum corneum, the non-viable selectively permeable barrier component of skin. Thus, exfoliation is a constitutive function of healthy normal skin.
The dermis is the inner layer of the skin containing blood capillaries, blood vessels, lymph vessels, hair follicles, and various glands. The dermis is composed of felted connective tissue containing elastin, collagen and fat. The dermis is divided into the upper, papillary layer and the lower, reticular layer.
The papillary layer of the dermis contains a large number of papillae, which rise perpendicularly from its surface. The papillary layer of the dermis also contains blood capillaries which carry nutrients to and remove waste from the dividing cells in the stratum basale.
The reticular layer of the dermis contains the blood vessels, sebaceous glands, arrector pili muscles, sensory nerve fibers, hair follicles, hair roots, pacinian corpuscles, hair root plexus, and eccrine sweat glands.
At the base of the dermis lies the subcutis, also known as the hypodermis or superficial fascia, composed primarily of adipose tissue.
In addition to exfoliation of dead epidermal cells, the skin is also capable of removing dermal content through a viable epidermis. This process, known as transepidermal elimination, allows the disposal of foreign material aberrantly implanted in skin (T. Y. Woo and J. E. Rasmussen, “Disorders of transepidermal elimination. Part 2,” Int. J. Dermatol. 24(6), 337-348 (1985)). Aberrant functioning of the transportation system can lead to several pathological skin conditions, such as Kyrle's disease, elastosis perforans serpiginosa, reactive perforating collagenosis, acquired perforating dermatosis, chondrodermatitis nodularis helices, and perforating folliculitis. These diseases share in common the physiological function of transepidermal elimination, albeit triggered by a stimulus that leads to a pathological state.
Electromagnetic radiation, particularly as produced by lasers, has been applied directly to the skin for treatment of dermatological conditions, for skin resurfacing, to reduce or eliminate rhytides, and to combat the effects of aging in the skin. Beyond treatment of the skin, electromagnetic radiation therapy has been used to increase the rate of wound healing, to reduce pain, to treat inflammatory conditions, as well as to reduce residual neurological deficits following stroke. When used for skin resurfacing, the effect of electromagnetic radiation on skin is primarily to heat the skin, producing thermal coagulation, cell necrosis, melting, welding and ablation, among other effects. Treatment with electromagnetic radiation can generally be divided into ablative and nonablative treatments. Ablation of the stratum corneum with electromagnetic radiation has been used for skin resurfacing and to perforate the skin to allow delivery of active substances and for the removal or monitoring of biological fluids or gasses. The use of nonablative electromagnetic irradiation of the skin has also been suggested to increase skin permeability by altering the lipids, water, and protein molecules present in the stratum corneum, by producing heat, and by producing pressure waves. However, nonablative electromagnetic radiation therapy has not been used for the treatment of unwanted skin conditions such as the presence of pigment in the dermis, a problem commonly seen by dermatologists treating melasma, tattoos, and post-inflammatory hyperpigmentation.